LEDs are increasing in importance on use as backlighting in liquid-crystal displays (LC displays). These novel light sources have a number of advantages over the conventional cold cathode fluorescence lamps (CCFLs), such as longer lifetime, potential energy saving, absence of harmful ingredients (such as mercury in CCFLs).
In the past, arrangements of LEDs which emit blue, green and red light were employed as backlighting source for LC TV applications. However, this multi-chip approach has some disadvantages: it is extremely difficult to combine three different chip materials and to ensure uniformity and stability of light parameters such as colour point.
pcLEDs (“phosphor-converted LEDs”) have therefore been introduced as light sources for use as backlighting. These usually comprise one green phosphor and one deep-red phosphor together with the blue light emission of an LED chip, which are matched in accordance with the transmission spectra of the coloured filter (transmission bands in the blue, green and red region of the spectrum). A set-up of this type theoretically facilitates colour gamuts which are much larger than the usual sRGB. Due to restrictions in the availability of suitable qualities, there is still a need for further optimised phosphors and/or phosphor mixtures having good colour gamuts.
Surprisingly, it has now been found that a sufficient colour gamut for backlighting applications can also be achieved by LEDs comprising a phosphor mixture which comprises three or more silicate phosphors of the formula IBawSrxCaySiO4:zEu  (I),in which all indices w, x, y and z are independent of one another, with the condition that within one compound the indices w, x, y and z add up to 2 (w+x+y+z=2),    and in which one or more silicate phosphors emit green light, one or more silicate phosphors emit yellow light and one or more silicate phosphors emit orange light and in which the weight ratio of green phosphor to yellow phosphor to orange phosphor is 1.8-4.0:0.1-4.0:0.8-3.0.
A first embodiment of the present invention is therefore a phosphor mixture comprising three or more silicate phosphors of the formula IBawSrxCaySiO4:zEu   (I),in which all indices w, x, y and z are independent of one another, with the condition that within one compound the indices w, x, y and z add up to 2 (w+x+y+z=2),    and in which one or more silicate phosphors emit green light, one or more silicate phosphors emit yellow light and one or more silicate phosphors emit orange light and in which the weight ratio of green phosphor to yellow phosphor to orange phosphor is 1.8-4.0:0.1-4.0:0.8-3.0.
A sufficient colour gamut means good coverage of the sRGB range; i.e. the coverage of the colour gamut triangle of the backlighting source including coloured filter in the CIE 1931 chromaticity diagram, where the triangle which includes the three sRGB colour points according to CIE 1931 should be as large as possible. Good coverage, which is suitable for TV applications, is achieved if more than 90% of the range, preferably more than 95% of the range, is covered.
Orthosilicates have a number of advantages over mixtures of deep-red nitrides:
They are much more widely available, less expensive and they enable matching of their fluorescence band in very small steps by slight changes in the composition of the elemental formula (Ca,Sr,Ba)2-xSiO4:Eux. This enables optimised matching of the transmission curves of the coloured filters.
In particular, it has been found, surprisingly, that by mixing of a plurality of orthosilicates, for example a greenish type with a yellow type and an orange type, the resultant mixture exhibits a broader band compared with a single orthosilicate phosphor. An LED having a broader emission band is important in order to ensure sufficient spectral power not only in the blue region (by LED chip emission) and green region (by phosphor emission), but also in the red region (by phosphor emission).
WO 02/054502 describes a phosphor mixture comprising two or three phosphors, preferably two silicate phosphors and a further phosphor (barium magnesium aluminate or strontium magnesium aluminate:Eu).
WO 2007/018569 describes a phosphor composition having at least two silicate-based phases comprising a first phase having a crystal structure which essentially corresponds to that of (Ca,Sr,Ba,Mg,Zn)2SiO4, and a second phase having a crystal structure which essentially corresponds to that of (Ca,Sr,Ba,Mg,Zn)3SiO5, where at least one phase of the composition comprises Mg and at least one of the phases of the composition comprises a dopant F, Cl, Br, S or N.
KR-2005-23990 describes an LED having a compression-moulding resin layer comprising a yellow-, green- and orange-based phosphor mixture with an amount ratio in the range from 70:20:10 to 95:4:1, preferably 75:16.8:8.2 or 80:13.6:6.4, in which at least one orthosilicate-based phosphor is used. The yellow phosphor forms the principal component of the mixture.